Our invention relates to a process for the fabrication of ceramic products, particularly of those of improved thermal conductivity composed principally of a nitride such as aluminum nitride (AlN). The process of our invention is well suited for the production of ceramic packages for circuit boards and integrated circuits although we do not wish our invention to be limited to this particular application.
It is a common knowledge among the ceramics specialists that some nitrides such as AlN are sinterable to maturity by hot pressing. The hot pressing is a process wherein pressure and heat are applied simultaneously at temperatures high enough for sintering to occur. This process has some drawbacks, however. One of these is the lack of adaptability for quantity production. Another is the difficulties involved in the fabrication of large size products.
These drawbacks of the hot pressing method are overcome by another known process that calls for the use of sintering aids such as calcium carbonate, CaCO.sub.3, aluminum oxide (alumina), Al.sub.2 O.sub.3, and yttrium oxide (yttria), Y.sub.2 O.sub.3. Any selected sintering aid is admixed with a desired nitride material in finely divided form. The admixture is molded into desired shape, and the moldings are sintered under atmospheric pressure. Liquefied at elevated temperatures, the sintering aid fills the interstices of the moldings and so aids in the sintering of the nitride material. The sintering aid serves the additional purpose of entrapping oxygen existing as an impurity substance in the nitride material.
The conventional method of making nitride ceramic shapes with use of sintering aids has its own weakness, however. The nitride material and the sintering aid in use reacts upon firing of the moldings and creates a substance that remains among the ceramic grains produced by sintering. It has been known that that intergranular substance reduces the thermal conductivity of the resulting ceramic products. The elimination of this undesired substance is essential for higher thermal conductivities of nitride ceramic products.